Phases of a Rocket's Flight

Are you new to model rocketry? Do you know what to expect when you build and fly a rocket kit? This article will try to help you by explaining what happens when you launch a model rocket. It will also point you in the right direction where you can find even more information to help make you into a rocketry expert.

Begin by looking at the picture to the right. If you are teaching others about rocketry, and you want to have a printable copy of the image, you can print out the one that appears in the Peak-of-Flight Newsletter #117. You will find that having a poster to hand out will be helpful. Please feel free to share it with your friends.

A model rocket is always ignited by "electrical ignition." That means that you need to have a device that starts the motor burning. This device is called a "launch controller." Basically, it consists of a battery, a switch, a safety key, and some wire. You can make your own if you have some basic electrical skills. There are plans for several different launch controllers in the booklet: Electronic Model Rocket Launcher Construction Plans

If you want to save time, you can buy a launch controller. They are usually sold as a set along with the launch pad. You can find a good one at called the Sky Launch System

Why electrical ignition? This is a common question. The answer has to do with "safety." Many new people think a fuse would be simpler. But that would sacrifice safety. Once a fuse is lit, there is no stopping it. If the rocket should tip over, or an airplane suddenly appear in the sky, you couldn't halt the launch.

The other item shown in the picture is a launch pad. Again, many new modelers don't understand the need for a launch pad that includes a launch rod. After all, the Space Shuttle doesn't have a big launch rod...

The purpose of the launch rod is to guide the rocket until it reaches sufficient speed where the fins take over and keep the rocket moving in a straight path. This is approximately 30 miles per hour.

By the way, the reason the Space Shuttle, and other large rockets don't have launch rods is because they have rocket engines that are steerable. In other words, the direction the rocket engine pushes controls the path of the rocket. Our model rockets have a fixed nozzle. They will only move in one direction. So we need a rod to keep the rocket moving in the "upward" direction. Without a rod, the model can easily tip over at lift-off, and come screaming right at you. So for safety, we have a launch rod that keeps the rocket pointed up.

The rest of the launch pad is needed to hold the rod, and to keep it from tipping over in

The actual device that starts the motor burning is the "starter." It looks like a match with wires coming off the tip. These wires are hooked up to the launch controller that we discussed earlier.

When the electrical current passes through the starter, it heats it up and causes it to burst into flame. This flame is what actually starts the propellant burning in the rocket motor.

Where do you get starters?

They come with the rocket motors.

How do you hook them up? It's pretty simple. Check them out!

Once the motor ignites, it begins to generate thrust. It is this thrust force that pushes the rocket into the air.

While the motor is making thrust, you'll normally see a flame coming out the back of the motor. Sometime it is hard to see because the rocket moves so fast. At the same time, the rocket motor is making a loud roar and a lot of thick dark smoke.

Phase 1 - Ignition and Lift-off

• Issue #1 - How are your flying skills?
• How to select rocket engines - a step-by-step guide
• Issue #38 - Selecting Rocket Motors (part 1)
• Issue #39 - Selecting Rocket Motors (part 2)
• Issue #40 - Selecting Rocket Motors (part 3)
• Issue #106 - Newton's Laws of Motion, and How Rockets Create Thrust.
• Issue #114 - How Black Powder Rocket Motors work.
• Issue #115 - How Composite propellant rocket motors work.

The propellant inside the motor burns quickly. In most motors, the propellant is consumed in less than three seconds, at which point "burnout" occurs. This means the motor is no longer producing a thrust force.

By the time the motor burns out, the rocket has already reached its top speed. It cannot go any faster from this point on.

Most people are surprised that burnout occurs at a very low altitude. While the rocket may reach hundreds or thousands of feet in the air, the burnout location on most rockets is about 50-80 feet in the air. If you want to predict when the burnout occurs during the flight, you can do this with the RockSim software.

Phase 2 - Engine Burnout

• Issue #33 - Finding Maximum Liftoff Weight # 1.
• Issue #34 - Finding Maximum Liftoff Weight #2.
• Issue #42 - Why do Rockets go Unstable?
• Issue #98 - How Multi-Stage Model Rockets work - Part 1.
• Issue #99 - How Multi-Stage Model Rockets Work - Part 2.

When the motor burns out, the rocket may be traveling hundreds of miles per hour. We don't want the parachute to come out of the model while it is going this fast. Otherwise, it will be ripped to shreds.

We want the model to coast upward and bleed off some speed. The period of time that starts at engine burnout, and ends when the parachute is ejected out of the rocket is called the
"Coast Phase."

Even though the rocket motor isn't making thrust, there is something happening inside of it. The special composition called the delay element (or delay grain) is burning at a slow rate. See how model rockets work!

It is obvious that something is going on, because there is still smoke coming out of the rocket motor. Maybe this is what causes confusion among new modelers that they think the motor keeps burning all the way until it reaches apogee (the highest point in the flight).

The smoke serves a purpose though. It allows us to track the rocket -- in other words, to follow its progress into the air. Sometimes the rocket moves so fast, that it is hard to follow with our eyes. So the smoke gives us a visual indication where the model is.

There is something worth mentioning. The smoke produced by the delay grain is not as dark or as thick as the smoke produced by the motor while it is producing thrust. The delay smoke is whiter, and wispy.

Phase 3 - Coasting Phase

• Issue #5 - What is an overstable rocket?
• Issue #59 - The Moving Target Called "Optimum Delay"

When the delay composition is done burning, it starts the "ejection charge" that is also built into the motor. This ejection charge burns quickly, and is directed inside the rocket. It's goal is to push off the nose cone, and eject the parachute out of the rocket.

Typically, we desire the ejection to occur right at apogee (the highest point in the trajectory of the rocket). It is at this point the rocket has slowed down to its minimum velocity. So when the chute comes out, it isn't hit by a huge gust of air.

The modeler controls when the ejection charge pushes out the parachute by proper motor selection. If you use a motor that has too long of a coast phase delay, the rocket will arc over, and will eject the chute while the rocket has built up some speed when it is coming back down to the ground. Likewise, too short of a delay will mean the rocket hasn't coasted to its highest point.

Phase 4 - Apogee and Ejection

• Issue #10 - Altitude Flying Strategies
• Issue #75 - Designing High-Altitude Rockets
• Issue #92 - How High Did the Rocket Go? - Part 1. Using Single Station Tracking
• Issue #93 - How High Did the Rocket Go? - Part 2. Two-Station Tracking

After the parachute has ejected, it fully inflates, and the rocket has begun its recovery phase.

Nothing much happens during the recovery phase. The model just drifts slowly to the ground under the canopy of the parachute. But it is at the mercy of any wind that is blowing. The stronger the wind, the further the model will drift away from the launch pad.

Because of this, modelers have searched for ways to keep the model from drifting out of sight. The most common thing they do is to switch from a parachute to a streamer. A streamer does the same thing as a parachute, but it falls faster, so it doesn't drift as far. There are other things a modeler can also do to prevent the rocket from drifting too far. You could cut a hole in the canopy of the chute, to make it fall faster. Similarly, you can tie the suspension lines together to reef the chute (to prevent it from opening fully). Again, this makes the rocket fall faster, so it doesn't drift as far during the recovery phase.

Phase 5 - Recovery

• Issue #4 - Dethermalizers for Parachute Models
• Issue #32 - Detecting and Using Thermals
• Issue #64 - Simulating Dual Deployment In RockSim (part 1)
• Issue #65 - Simulating Dual Deployment In RockSim (part 2)

All these articles can be found on our archive.

With the exception of the recovery phase, the flight of the rocket is controlled by the rocket motor. For a new modeler just starting out, proper motor selection is a very important part of the launch process.

Fortunately, most new modelers choose kits to fly. The manufacturer of the kit will make a recommendation on which motor to choose for the flight. As long as you follow this recommendation, your flight should be a success.